Rapid progress in electrical, electronic, information communication technologies combines information communication systems, such as computers, switches, transmission equipment, repeaters, and servers, closely and organically. These information communication systems cannot secure reliability without very stable power supply. In other words, equipment requiring high-level digital information processing, such as cutting-edge industrial equipment, medical equipment, computers, diverse financial equipment, office automation equipment, precise control equipment, and information communication equipment, is very sensitive to a voltage variation such as sagging, swelling, outage, overvoltage, low voltage, and voltage unbalance.
Undesired voltage variation may degrade the quality of electrical power, electronic equipment may malfunction, stop operation, or even cause fire. To cope with power failure or a change in voltage, devices capable of preventing abnormality in power source such as voltage variation, frequency variation, temporary electricity failure, and overvoltage and continuously supplying stable power source. A representative one of such devices is a uninterruptible power supplier (UPS).
Also, more than two UPSs are set up for a load that may cause a great deal of economic damage when temporary load accident occurs, and a static transfer switch is set up in the output end of each UPS to prepare for a case where any one UPS goes out of order.
FIG. 1 is a schematic diagram showing a structure of a typical power supplying apparatus supplying power to a load by using two UPSs. FIG. 2 is a schematic diagram describing a structure of a typical power supplying apparatus supplying power to a load by using two UPSs with a static transfer switch.
The static transfer switch is set up between the output ends of first and second UPSs (Source1 and Source2) of the power supplying apparatus shown in FIG. 1. The static transfer switch supplies output power source of any one between the first and second UPSs (Source1 and Source2) to a load so as to supply power source stably.
For this, the output ends of the UPSs (Source1 and Source2) are connected to the input ends of silicon controlled rectifier (SCR) switches (SCR1 and SCR2) through the static transfer switch, and the output ends of the SCR switches (SCR1 and SCR2) are commonly connected to the load. The silicon controlled rectifier is also called thyristor. First and second SCR switches (SCR1 and SCR2) are connected to first and second circuit brakes (CB1 and CB2) in parallel, respectively. The first and second circuit brakes (CB1 and CB2) cut off a circuit manually.
The first and second circuit brakes (CB1 and CB2) are manipulated by a human being to check a system when an SCR switch is broken down and examined or in case of emergency.